The present invention relates to a method for the formation of a combustible fuel/air mixture in the combustion chamber of a direct-injection internal combustion engine with an injection nozzle, which has a closure body and via which the fuel is introduced in at least two partial quantities into the combustion chamber.
A method for the formation of a combustible fuel/air mixture is described, for example, in German Published Patent Application No. 196 42 653. According to this method, a combustible fuel/air mixture can be formed in the cylinders of direct-injection internal combustion engines by a process in which, after the exposure of a nozzle opening by raising a valve member from a valve seat that includes the nozzle opening, fuel is injected by an injector into each combustion chamber delimited by a piston. In order to allow internal mixture formation that is optimized for consumption and emissions at every operating point of the entire engine map under all operating conditions of the internal combustion engine, especially in stratified-charge mode, provision is made for the opening stroke of the valve member and the injection time to be variable. In this case, the geometry of the jet may be altered by coking of the injection valve, and increased emissions of soot due to poor mixture formation in stratified lean-mixture operation and a reduction in the reliability of ignition due to the changing quality of the mixture at the spark plug and due to coking in stratified lean-mixture operation are possible. In addition, there are increased proportions of unburnt fuel due to dilution of zones of the mixture in stratified lean-mixture operation. In addition, there is wetting of the spark plug and hence failure thereof due to soot deposition, increased emissions of pollutants owing to incomplete combustion of the mixture at the spark plug due to random scatter of the injection jet and collapse of the injection jet under the back pressure in the combustion chamber in stratified lean-mixture operation, i.e., an increased likelihood of misfires.
It is therefore an object of the present invention to provide a method that ensures reliability of ignition and avoids coking of the spark plug at all operating points.
The above and other beneficial objects of the present invention are achieved by providing a method in which the closure body of the injection nozzle can be moved into its closed position after the injection of each partial quantity. This ensures that the fuel input or the two fuel pulses are injected in a defined manner at the respective instant and thus make a significant contribution to optimum mixture formation. Closing the nozzle opening without reducing the fuel pressure applied significantly improves the respective fuel pulse.
It may be advantageous for this purpose that the first partial quantity is greater than the second partial quantity, 70% to 99% or 80% to 99% of the total quantity of fuel being introduced first, and the remainder being introduced after 0.05 ms to 0.4 ms or 1xc2x0 of crank angle to 5xc2x0 of crank angle and the injection cycle being ended between 50xc2x0 of crank angle and 5xc2x0 of crank angle before TDC (Top Dead Center). The main quantity of fuel initially introduced is prepared in an optimum manner by the extended mixture formation time before ignition and by the second pulse including the remaining quantity of fuel, and an undiluted combustible fuel/air mixture is formed.
According to one example embodiment of the present invention, the fuel may be introduced as a fuel cone and may produce a toroidal vortex at the end of the cone envelope in the region of a piston. Thus, an undiluted combustible fuel/air mixture that ensures initiation of ignition may be formed in the region of the spark plug. Inside and outside the fuel cone, the toroidal vortex carries the fuel introduced into the other regions of the combustion chamber and particularly into the region of the spark plug.
The nozzle opening of the injection nozzle may be disposed at a distance (A) of 1 mm to 8 mm from a combustion-chamber roof and at a distance (B) of 10 mm to 15 mm from a spark plug, the injection pressure of the injection nozzle varying between 100 bar and 300 bar or between 150 bar and 250 bar. The fuel jet emerging from the injection nozzle may be formed approximately conically and may include a constant jet angle xcex1 that is independent of the position or location of the closure element. The form of fuel jet required for optimum mixture formation, i.e., a toroidal vortex, is thereby achieved. The position of the spark plug and the position of the fuel jet may define the formation of the optimum mixture.
According to an example embodiment of the present invention, the jet angle xcex1 may be 10% to 50% or 20% to 40% smaller than the angle xcex2 of the combustion-chamber roof. It is thus possible to prevent wetting of the combustion-chamber roof and to prevent the toroidal vortex from striking the combustion-chamber roof.
The fuel jet may include at least one or one inner and one outer toroidal vortex at the end of its cone envelope in the region of the piston. Optimum mixture formation is thus achieved throughout the combustion chamber.
According to the present invention, the closure element may be mounted in a coaxially rotatable manner and may be moved axially by between 0 xcexcm and 80 xcexcm or 10 xcexcm and 50 xcexcm into the combustion chamber at any time via the piezoelectric element. The rotatable closure body thus contributes a circumferential velocity component to the fuel jet or fuel cone, thus improving mixture formation and fuel input.
The closure body may include a conical sealing surface with an angle xcex4 of between 70xc2x0 and 90xc2x0 or between 70xc2x0 and 85xc2x0, and a housing of the injection nozzle may include a curved, parabolic or conical outlet cross section, thus forming the sealing seat or the sealing surface of the injection nozzle. Thus, the gap or nozzle opening tapers continuously towards the outlet with a curved or parabolic outlet cross section, and the fuel jet is thus accelerated continuously up to its emergence. In this arrangement, the fuel jet has a jet angle xcex1 that is independent of the position of the closure element.